net:sunxi-gmac: add gmac support

The commit itself comes from

https://github.com/cubieboard/linux-sunxi/tree/cubie/sunxi-3.4/

IRC user jemk found out the IP itself comes from DesignWare. It turns
out it is supported already in both the mainline (stmmac) and the
U-Boot (designware). These existing drivers should be made to work
with Allwinner chips. This driver is solely for the purpose of the
reference and will not be improved.